1. Field of the invention
The present invention concerns an asynchronous motor power supply control system.
2. Description of the prior art
Asynchronous motors are usually supplied with power from an AC line voltage via a variable speed drive. The variable speed drive is a frequency converter including a rectifier bridge connected to the AC line voltage, a DC intermediate circuit including a capacitor, and an inverter with solid-state switches controlled by a pulse modulation device and connected to the phase windings of the motor.
A motor operating as a generator generates a braking torque which can be used to compensate system losses.
On braking, the motor acts as an energy recovery device and charges the capacitor of the DC circuit. The charge on the capacitor increases the DC voltage or bus voltage (V.sub.bus) of this circuit. This voltage must not reach the maximum voltage that the capacitor and the switches are rated to withstand. The voltage is limited to a threshold voltage V.sub.sup by reducing the current in the motor.
In the event of a short-term interruption of the AC line voltage, the motor can be allowed to "free wheel" by turning off all the transistors, but the motor is then out of control. The loss of control over the speed makes restarting after the interruption difficult. Unless specific precautions are taken, the transient conditions when control is not optimum can cause high overcurrents. It is therefore preferable to retain control of the motor by applying braking in the event of a short-term interruption of the AC line voltage.